1. Field of the Invention
The present invention relates to catalysts composed of
a) a magnetizable core having a diameter of from 5 to 100 nm, PA0 b) which may be coated with a binder and PA0 c) which carries catalytically active metals or metal compounds on its surface.
The present invention furthermore relates to a process for the preparation of these catalysts, their use and a method for removing these catalysts from reaction solutions.
2. Description of the Related Art
Reactions in solution with homogeneous or suspended catalysts are widely encountered in the chemical industry. However, the removal and, if required, recycling of these catalysts presents difficulties in many cases. If catalysts dissolved in the reaction solution, ie. homogeneous catalysts, are used, the products are usually separated from the catalysts by distillation and the catalysts remain in the bottom product of the distillation. The catalysts are frequently deactivated during this procedure and can be reused only after being worked up by a complicated process. In the case of thermally unstable products, this method of removal by distillation leads to a deterioration in the product quality owing to decomposition reactions. In these cases, the catalyst may alternatively be removed, for example, by extraction or by adsorption, for example on active carbon. Attempts have also been made to achieve separability of product and catalyst by heterogenization, ie. by binding the catalysts to finely divided substances which are insoluble in the reaction mixture. However, the filterability of such heterogenized catalysts is often poor.
In the case of suspension catalysts, the highest catalytic activity is frequently achieved with small catalyst particles having a high specific surface area. Owing to the particle size, these particles, too, are difficult to remove from the reaction mixture by simple filtration, so that the technically complicated separation methods described above have to be used.
A possible method for removing metallic suspension catalysts containing the magnetic elements iron, cobalt and nickel entails removal in a magnetic field (magnetic filter) (Journal of Magnetism and Magnetic Materials 85 (1990), 285). However, this method is limited to magnetic metals and, in the case of larger particles, also has the difficulty that the particles agglomerate to form larger particles as a result of permanent magnetization.
The immobilization of enzymes by binding to magnetic particles is described, for example, in EP-A 125995. However, the use of these enzymes is limited to mild reaction conditions under which the enzymes bound in this manner are chemically stable.